A previous thread has much the same discussion.
The 2011 posting below has similar information.

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Subject: Lyr Add: THE RED RIVER VALLEY
From: Taconicus - PM
Date: 05 Dec 11 - 11:56 AM

I've been able to obtain a copy of Edwin Piper's original manuscript of the lyrics to Red River Valley from the Edwin Ford Piper Collection of the University of Iowa Libraries with the help of Jacque Roethler, Special Collections Assistant at the University of Iowa in Iowa city (not to be confused with Iowa State University in Ames, Iowa).

The song is of course in the public domain, and since the manuscript is unpublished, the copyright expired 70 years after the death of the author, which would be 2009 since Edwin Piper died in 1939. Accordingly, the manuscript is now in the public domain. I've just posted a copy on the Web, HERE.

Nemaha and Harlan are counties in Nebraska (Edwin Piper was born in Auburn, Nebraska). I believe the notations at the bottom of the manuscript therefore refer to the places and years that those folk song lyrics were collected (Piper was an avid collector of folksongs). If the song was written, as is supposed, shortly after the Red River rebellion (1869-70) in Manitoba, this would mean that these lyrics (as recorded in 1879) must be very close to the original. In any case, this indicates that these lyrics were collected long before the 1896 version by James Kerrigan listed in the Mudcat lyrics (DT) section.

Edwin Piper likely did not collect these himself since he was not born until 1871, so these were probably collected during his childhood by someone he knew and given to him later. Perhaps it was a family member, perhaps his mother or father, who was also interested in folk music and collected them?

I debuted the song at the Hudson Valley Folk Guild this past Saturday, and it was a big hit - no one had heard all those lyrics before. It's one of those "classic American folksongs" (a category in which I include songs from Canada) of which everyone is familiar, but of which practically no one has heard more than a single verse and chorus. I added two words to the lyrics for the performance since the second line of the fourth verse from the manuscript does not scan, nor does it rhyme as the rest of the song does, and therefore I consider it suspect as probably not being the original lyric. I replaced that line (for the performance) with "Don't forget the sweet hours so free." I also sang the chorus after the second and fourth verses only.

The Red River Valley

From the valley they say you are going,
I shall miss your bright eyes and fair smile;
But alas, you take with you the sunshine
That has brightened my pathway awhile.

      Chorus:
      Then consider awhile ere you leave me
      Do not hasten to bid me adieu
      But remember the Red River Valley
      And the heart that has loved you so true.

Do you think of the valley you're leaving,
How lonesome and dreary 'twill be?
Do you think of the heart you have broken
And the sorrow that o'ershadows me?

It is a long time I've been waiting
For the words that you never would say,
But alas, all my hopes they have vanished
For they say you are going away.

When you go to your home by the ocean,
Don't forget the sweet hours so free,
That we spent in the Red River Valley
And be true to your promise to me.

The fair maiden prays for her lover
To the spirit that rules o'er the world
May his pathway be covered with sunshine
Is the prayer of the Red River girl.

Nemaha 1879, Harlan 1885
The Edwin Ford Piper Collection, The University of Iowa Libraries, Iowa City, Iowa
(The italicized words in the lyrics were added by the author of this post, 2011)